Please share some more information. Are you going alone, with family or friends? What is your budget? What are you planning to go for? Shopping, beaches, adventure, history, night life? There is a lot to do in Thailand and every area has its own speciality, unless you know what you want to experience, you can't plan properly. A rough guide to give you an idea about the variety:
Bangkok: A proper cosmopolitan city with everything to offer i.e. shopping, food, culture, history, nightlife etc.
Pattaya: Entertainment and nightlife.
Phuket: A mix of entertainment and nature/adventure.
Koh Samui: Good for relaxing and honeymoon.
Koh Phi Phi: Night life and some sightseeing.
Koh Phangan: Clubbing nightlife only.
Koh Tao: Diving and snorkeling paradise.
Chang Mai: Beautful trekking areas.